Problem Description:
Given an array with N capacity, which store N integers from 1 to N-1 with one and only one integer
occurs twice, please find that repeated integer. Note, limit the time complexity to O(n).
Solution:
int find(int a[], int N)
{
int i, sum, base_sum;
sum = 0; // the sum of those N integers.
for (i = 0; i < N; i++) {
sum += a[i];
}
base_sum = (N*(N-1))/2; // the sum of all the subscripts.
return(sum - base_sum);
}